While I have absolutely no problem with Megs having more mobile alt. modes, I think a gun\canon mode could work even in modern media with the right setting. For an example, I could imagine a more aristocratic, elitist take on Megatron who believes travelling himself is beneath him, and having himself be carried by his subordinates would be a sort of dominance-asserting power move. Kind of a "We don't drive ourselves, that is for the help" mentality. Bonus points if his alt. mode comes with other advantages, like somehow being able to shoot stronger blasts or take control over anyone wielding him.